The present invention relates generally to surgical implant devices and more particularly to intraocular lenses.
As persons age, their eyes typically loose the ability to change focus quickly and to clearly focus on close, distant and intermediately-positioned objects. Because of this loss of accommodation in the human eye, persons reaching their early forties often find it necessary to wear reading glasses to clearly focus on closely-positioned objects.
An even more pronounced loss of accommodation typically occurs when the crystalline lens is replaced by a conventional intraocular lens. Such replacement is performed, as is well known, in response to the formation of cataracts in the crystalline lens or when the crystalline lens is damaged by disease or injury. It has been recognized that with time the ciliary muscle of the eye develops the ability to move the intraocular lens back and forth along its center axis, thereby changing the focal point of the light rays passing through the intraocular lens and reaching the retina. Less than one diopter of power change is believed to occur pursuant to this movement. This minimal accommodation is insufficient to permit an individual to focus on close and distant objects, since 3 to 6 diopters of power change is typically required to achieve this range of change in focus.
In an attempt to overcome loss of accommodation in the eye, bifocal intraocular lenses have been developed. These lenses include a relatively thick center section comprising a minor portion of the entire diameter of the lens and a relatively thin peripheral section surrounding the center portion and having a thickness substantially equal to that of conventional intraocular lenses. The thicker center section has a focal power that is selected to permit the eye to focus on close objects and the thin peripheral section has a focal power that is selected to permit the eye to focus on distant objects.
When an eye containing such a bifocal intraocular lens is focused on a close object, the iris of the eye closes down. As a result, only the light rays passing through the thick center section of the lens reach the retina, whereby the closely-positioned object appears in focus. On the other hand, when the eye is focused on distant objects, the majority of light rays passing through the bifocal intraocular lens pass through the peripheral section thereof (because the peripheral section comprises the majority of the bifocal intraocular lens), whereby the distant objects appear in focus. Because only a small percentage of the light rays traveling through the bifocal lens in an eye focused on a distant object pass through the thick center section, the mind is able to ignore these rays with the result that these differently-focused light rays are not visible.
Although the bifocal intraocular lens has improved accommodation capabilities as compared to a conventional intraocular lens, the former does not provide the type and degree of accommodation provided by the crystalline lens of a young person's eye. Specifically, the bifocal intraocular lens refracts incoming light rays such that viewed objects are in focus at substantially only two focal lengths, typically close and distant. By way of contrast, a young person's eye varies the focus in analog fashion between close and distant objects so that objects at all focal lengths (assuming the person has 20/20 vision) are in focus. Hence, it is a significant drawback of the bifocal intraocular lens that it cannot provide the continuum of focal powers required to permit clear focusing on close, distant and intermediately-positioned objects.